This invention relates to linear accelerators in general and relates more particularly to a LINAC that is powered by pulses of energy rather than by RF power.
In the field of particle accelerators, circular machines are used widely because of the convenience involved when imparting energy to particles traveling in a repetitive circular orbit. However, at relatively high energy levels, say above 100GeV, because of synchrotron radiation losses the use of circular machines for electron acceleration is just about precluded. Thus, the possibility of utilizing linear accelerators (LINAC) is being explored more than ever.
While switch power or pulse power LINACS are known, for the most part, particle accelerators of the prior art have utilized RF switching power compression schemes. A switch power LINAC structure is disclosed by W. Willis in an article appearing on pages 166-174 of the Proceedings of the SAS-ECFA-INFN Workshop, Frascati, Sept. 1984. The Willis device consists of a set of parallel discs each having a hole through which the electron beam is accelerated by an electromagnetic wave which is injected uniformly in appropriate phase at the periphery of the discs. The wave is compressed spatially as it travels towards the holes at the center of the discs.
In a Willis type structure the energizing wave front that is injected must be uniform around the periphery of the discs or else transverse fields will be experienced by the particles that are being accelerated. Further, the electric field obtained at the center of the disks depends upon the ratio g/tc (g is the distance between the two disks, c the speed of light, t the rise-time of the pulse). The dependence is such that in order to obtain a substantial gain in electric field at the disk's center, the injected pulse must have a fast risetime (faster than g/c). This requires a costly switching system combined with fast risetime requirements, high peak power and a circular switching configuration. As will hereinafter be seen, the LINAC constructed in accordance with teachings of the instant invention does not require extremely fast risetime for the input power pulses in order to achieve high gradients efficiently, and recovery of energy that is not consumed by particle acceleration may be achieved without interfering with input power switching. Further, to achieve equal accelerating gradients, in most cases a radial line device of the Willis type requires higher energy per unit length than does the LINAC of the instant invention.